The CurrencyID for a Supplier is inherited by transactions for that Supplier.
In a single-currency environment that Currency is always the base currency. Since it never changes it is not shown on the data entry forms.
When Currency Manager is installed, that Currency is shown for reference purposes on the data entry forms for “Enter Payables transactions” and “Recurring Payables transactions”.
When these transactions are posted to Accounts Payable the system uses the CurrencyRate for the transaction date in the CurrencyRate table.